6. Perpetual Change

The solar corona - loops, holes and unexpected heat

Total Eclipse Of The Sun

Still higher, above the chromosphere, is the corona, from the Latin word for “crown”. The corona becomes momentarily visible to the unaided eye when the Sun’s bright disk is blocked out, or eclipsed, by the Moon and it becomes dark during the day. During such a total solar eclipse, the corona is seen at the limb, or apparent edge, of the Sun, against the blackened sky as a faint, shimmering halo of pearl-white light (Fig. 6.1). But be careful if you go watch an eclipse, for the light of the corona is still very hazardous to human eyes and should not be viewed directly.

A total eclipse of the Sun occurs when the Moon passes between the Earth and the Sun, and the Moon’s shadow falls on the Earth. In an incredible cosmic coincidence, the Moon is just the right size and distance to blot out the bright photosphere when properly aligned and viewed from the Earth. In other words, the apparent angular diameters of the Moon and the visible solar disk are almost exactly the same, so that under favorable circumstances the Moon’s shadow can reach the Earth and cut off the light of the photosphere. The Moon then acts just like your thumb with your arm stretched out and pointed at the Sun. At that distance from your eye, a thumb subtends an angle of about 30 arc-minutes, roughly the same as that of the Moon and the Sun.

A total eclipse does not happen very often. Since the Moon and the Earth move along different orbits whose planes are inclined to each other, the Moon only passes directly between the Earth and the Sun about three times every decade on average. Even then, a total eclipse occurs along a relatively narrow region of the Earth’s surface, where the tip of the Moon’s shadow touches the Earth (Fig. 6.2). At other nearby places on the Earth, the Sun will be partially eclipsed, and at more remote locations you cannot see any eclipse.

The low corona, that is close to the photosphere, shines by visible sunlight scattered by electrons there. This electron-scattered component of the corona’s white light has been named the K corona. It emits a continuous spectrum without absorption lines, and the K comes from the German Kontinum.

The amount of observed coronal light is proportional to the electron density integrated along the line of sight, so we can use observations of the K corona to infer the density of electrons there (Fig. 6.3). At the base of the corona there are almost a million billion (1015) electrons per cubic meter. These coronal electrons are so tenuous and rarefied that a million, billion cubic meters would only weigh one kilogram. Since protons are 1836 times more massive than electrons, they supply most of the corona’s mass. Since the corona is made from hydrogen atoms, there is one proton for every electron in the hot gas. The mass density in the low corona is about 10-12 kilograms per cubic meter.

The F corona is the more distant component of the corona’s white light. It extends from about two or three solar radii to far beyond the Earth (Fig. 6.3), and is caused by sunlight scattered from solid dust particles in interplanetary space. Unlike the K corona, the spectrum of the F corona includes dark Fraunhofer absorption lines, so the F stands for Fraunhofer. The faint light of the F corona is not polarized, with any preferred direction, but the K corona is, so polarization is another way to distinguish between the two components.

White-light coronal photographs show that the electrons can be confined within helmet streamers (Fig. 6.4), which are peaked like old-fashioned, spiked helmets once fashionable in Europe. At the base of helmet streamers, electrified matter is densely concentrated within magnetized loops rooted in the photosphere. Further out in the corona, the streamers narrow into long stalks that stretch tens of billions of meters into space. These extensions confine material at temperatures of about two million degrees Kelvin within their elongated magnetic boundaries.

Near the maximum in the activity cycle, the shape of the corona and the distribution of the Sun’s extended magnetism can be much more complex. The corona then becomes crowded with streamers that can be found close to the Sun’s poles (Fig. 6.5). At times of maximum magnetic activity, the width and radial extension oaf a streamer is smaller and shorter than at activity minimum. Near solar maximum, the global dipolar magnetic field of the Sun swaps its north and south magnetic poles, so a much more volatile corona can exist then.

Natural eclipses of the Sun occur every few years, and can then be seen from only a few, often remote places on the globe. So, scientists decided to make their own artificial eclipses by putting occulting disks in their telescopes to mask the Sun’s face and block out the photosphere’s intense glare. Such instruments are called coronagraphs, since they let us see the corona. The first coronagraph was developed in 1930 by the French astronomer Bernard Lyot, and the corona is now routinely observed with coronagraphs at mountain sites.

As Lyot realized, coronagraph observations are limited by the bright sky to high-altitude sites where the thin, dust-free air scatters less sunlight. He therefore installed one at the Pic du Midi observatory in the Pyrennes. The higher and clearer the air, the darker the sky, and the better we can detect the faint corona around the miniature moon in the coronagraph. They work best in space, where almost no air is left. Modern solar satellites, such as the Solar and Heliospheric Observatory or SOHO, use coronagraphs to get clear, edge-on views of the corona from outside our atmosphere (Fig. 6.6). Such satellites also use ultraviolet and X-ray telescopes to view the low corona across the face of the Sun, a development that followed the realization of the corona’s million-degree temperature.

The Corona’s Searing Heat

The solar corona defies expectations, for it is hundreds of times hotter than the underlying photosphere, which is closer to the Sun’s energy generating core. The Sun’s temperature rises to more than one million degrees Kelvin just above the photosphere at a temperature of 5,780 degrees Kelvin. Heat simply should not flow outward from a cooler to a hotter region. It violates the laws of thermodynamics, the branch of physics that deals with the movement and transfer of heat. These laws indicate that it is physically impossible to transfer thermal energy by conduction from the underlying photosphere to the much hotter corona. The high temperature of the corona also defies common sense; after all, when you sit farther away from a fire it becomes colder, not hotter. It is as if a cup of coffee was put on a cold table and suddenly began to boil.

The linkage between the chromosphere and the corona occurs in a very thin transition region, less than 100 thousand meters thick, where both the density and temperature change abruptly (Fig. 6.7). In the transition region, the temperature shoots up from 10,000 to more than a million degrees Kelvin, but the density decreases as the temperature increases in such a way to keep the gas pressure spatially constant. The corona then thins out and slowly cools with increasing distance from the Sun.

The Ultraviolet And X-Ray Sun

For studying the corona and identifying its elusive heating mechanism, scientists look at ultraviolet and X-ray radiation. This is because very hot material – such as that within the corona – emits most of its energy at these wavelengths. Also, the photosphere is too cool to emit intense radiation at these wavelengths, so it appears dark under the hot gas. As a result, the hot corona can be seen all across the Sun’s face, with high spatial and temporal resolution, at ultraviolet and X-ray wavelengths.

Since ultraviolet and X-rays are partially or totally absorbed by the Earth’s atmosphere, they must be observed through telescopes in space. This has been done using a soft X-ray telescope on the Yohkoh spacecraft, and with ultraviolet and extreme ultraviolet telescopes aboard the SOlar and Heliospheric Observatory, or SOHO for short, and the Transition Region and Coronal Explorer, or TRACE.

Observations at different temperatures can also be used to focus on different layers of the solar atmosphere. As an example, the spectral line of singly ionized helium, He II, at 30.4 nanometers, is thought to be formed at 60,000 degrees Kelvin, and it is therefore used to image structure in the lower part of the transition region, near the chromosphere (Fig. 6.8). The Sun is mottled all over in this ultraviolet perspective, like a cobbled road or a stone beach. Each stone is a continent-sized bubble of hot gas that flashes on and off in about 10 minutes. The whole Sun seems to sparkle in the ultraviolet light emitted by these localized brightening, known as blinkers. About 3,000 of them are seen erupting all over the Sun, including the darkest and quietest places at the solar poles.

Images at extreme-ultraviolet and X-ray wavelengths have shown that the hottest and densest material in the low corona is concentrated in magnetic loops. Indeed, Yohkoh’s soft X-ray images have demonstrated that the entire corona is stitched together by thin, bright, magnetized loops that shape, mold and constrain the million-degree gas (Fig. 6.9). Wherever the magnetism in these coronal loops is strongest, the coronal gas in them shines brightly at soft X-ray wavelengths (Fig. 6.10, Fig 6.11).

High-resolution TRACE images at the Fe IX, Fe XII and Fe XV lines, respectively formed at 1.0, 1.5 and 2.0 million degrees, have demonstrated that there is a great deal of fine structure in the coronal loops (Fig. 6.12). They have pointed toward a corona comprised of thin loops that are naturally dynamic and continually evolving. These very thin loops are heated in their legs on a time span of minutes to tens of minutes, after which the heating stops or changes, suggesting the injection of hot material from somewhere near the loop footpoints in the photosphere or below. The erratic changes in the rate of heating forces the loops to continuously change their internal structure.

The ultraviolet and X-ray emission of the Sun vary significantly over the 11-year cycle of magnetic activity. The ultraviolet emission doubles from activity minimum to maximum, while the X-ray brightness of the corona increases by a factor of 100. At the cycle maximum, when the sunspots and their associated active regions are most numerous, bright coronal loops dominate the X-ray Sun; at activity minimum the bipolar sunspots and their connecting magnetic loops have largely disappeared, and the Sun is much dimmer in X-rays (Fig. 6.13). However, the corona still stays hot at a minimum in its 11-year activity cycle when active regions go away; the million-degree gas is just a lot more rarefied and less intense.

Not all magnetic fields on the Sun are closed loops. Some of the magnetic fields extend outward, within regions called coronal holes. These extended regions have so little hot material in them that they appear as large dark areas seemingly devoid of radiation at extreme-ultraviolet and X-ray wavelengths (Fig. 6.14). Coronal holes are nearly always present at the Sun’s poles, and are sometimes found at lower solar latitudes. They are routinely detected by instruments aboard the SOHO, TRACE and Yohkoh spacecraft.

Solving The Heating Crisis

The temperature of the million-degree corona is not supposed to be hotter than the cooler photosphere immediately below it. Heat should not emanate from a cold object to a hot one any more than water should flow uphill. For more than half a century, scientists have been trying to identify the elusive heating mechanism that transports energy from the photosphere, or below, out to the corona. We know that sunlight will not do the trick, for the corona is transparent to most of it.

Magnetic waves provide a method of carrying energy into the corona. The ever-changing coronal loops are always being jostled, twisted and stirred around by motions deep down inside the Sun where the magnetism originates. A tension acts to resist the motions and pull the disturbed magnetism back, generating waves that propagate along magnetic fields, somewhat like a vibrating string. They are often called Alfvén waves after Hannes Alfvén who first described them mathematically. He pioneered the study of the interaction of hot gases and magnetic fields, receiving the Nobel Prize in Physics in 1970 for his discoveries in it.

However, once you get energy into an Alfvén wave, it is difficult to get it out. So there may be a problem in depositing enough magnetic-wave energy into the coronal gas to heat it up to the observed temperatures. Like radiation, the Alfvén waves seem to propagate right through the low corona without being noticeably absorbed or dissipated there.

Magnetic loops can heat the corona in another way – by coming together and releasing stored magnetic energy when they make contact in the corona. Internal motions twist and stretch the magnetic fields, slowly building up their energy. When these magnetic fields are pressed together in the corona, they merge, join and self destruct at the place where they touch, releasing their pent-up energy to heat the gas.

The SOHO spacecraft has provided direct evidence for such a transfer of magnetic energy from the solar photosphere into the low corona. Images of the photosphere’s magnetism, taken with SOHO, reveal tens of thousands of pairs of opposite magnetic polarity, each joined by a magnetic arch that rises above them. They form a complex, tangled web of magnetic fields low in the corona, dubbed the magnetic carpet (Fig. 6.15). The small magnetic loops rise up out of the photosphere and then disappear within hours or days. But they are continuously replenished by the emergence of new magnetic loops, rising up to form new magnetic connections all the time and all over the Sun.

The idea of powerful energy release during magnetic reconnection is not a new one. It was proposed decades ago to account for sudden, brief, intense explosions on the Sun, called solar flares, that can release energies equivalent to billions of terrestrial nuclear bombs. Converging flows in solar active regions apparently press oppositely-directed field lines together, releasing magnetic energy at the place that they join. The new, reconnected field lines can snap apart, accelerating and hurling energetic particles out into interplanetary space and down into the Sun.

Such bi-directional, collimated and explosive jets of material have been observed in ultraviolet images of the chromosphere outside active regions (Fig. 6.15). The magnetic interaction of coronal magnetic loops, driven together by underlying convective motions, also energizes at least some of the bright “points” found in X-ray images of the Sun. Unlike sunspots and active regions, the X-ray bright “points” are uniformly distributed over the Sun, appearing at the poles and in coronal holes, some almost as large as the Earth. Hundreds and even thousands of them come and go, fluctuating in brightness like small flares, apparently energized by magnetic reconnection.